Attitude control systems are utilized on space vehicles and space platforms to maintain the attitude and orientation of the vehicle or platform in space. Typically, these attitude control systems include a plurality of reaction wheel assemblies which are arranged together in such a manner that the speeds of the wheels may be varied to produce a resultant torque which changes the attitude of the vehicle. Deviations in the attitude of the vehicle are sensed and a corresponding torque change is made in one or more of the reaction wheel assemblies to bring the vehicle back to its desired attitude and orientation in space.
As can be appreciated, in a telescope experiment, it is necessary that the telescope be oriented precisely in space. This necessitates that the attitude of the space vehicle or platform be precisely maintained in order to maintain pointing accuracy of the telescope. However, the problem occurs that disturbance forces and torques are produced by the reaction wheel assemblies due to their rotor imbalance, misalignment, bearing defects, and bearing cage instability. These forces and torques, although quite small, can cause deviations in the attitude of the vehicle or excite resonances in the spacecraft structure, resulting in jitter of the telescope mirror. A need therefore arises for testing these reaction wheel assemblies in operation to measure and correct these disturbance forces prior to launch of the vehicle.
In the past, these reaction wheel assemblies have been tested on platforms supported by means of soft flexures. A disturbance force in the reaction wheel results in motion of the platform on its flexures. The acceleration due to this motion is measured by accelerometers mounted on the platform. The accelerations are converted to forces by multiplying the accelerometer output by the mass of the body on the flexures. These measuring devices must be designed to measure forces emitted from a rotating assembly to levels as low as 0.0002 pounds since these vibrational operating forces are usually of minute proportions. However, the forces are of a sufficiently significant proportion to change the attitude of a vehicle in space.
The problem has occured in the prior testing devices that due to system dynamics, the masses of the elements being accelerated are not known accurately which causes difficulty in calculation of the forces from the accelerometer readings. Furthermore, the mounting of the test platform on soft flexure members such as spring biased cantilevered arms has not provided sufficient isolation to allow measurement of forces to as low a level as necessary. Even where load cells are utilized, it is not possible to determine what percentage of the emitted forces are transmitted through the load cell or through the flexure mount.
Accordingly, an important object of the present invention is to provide a device for measuring the emitted vibrational forces produced by a reaction wheel assembly due to rotor imbalances, misalignment, bearing defects, cage instability and other minor elements. By accurately measuring these vibrational forces, the source of these forces can be determined and corrected in the wheel assembly.
Still another important object of the present invention is to provide an emitted rotation force measuring device and method in which these forces are more accurately channeled through a sensing axis of a sensing device to provide a more accurate reading of the forces and identification of their source components.
Still another important object of the present invention is to provide an emitted vibrational force measuring device and method in which a mounting carriage for a reaction wheel assembly is supported on generally frictionless air bearings and attached to a base plate by means of piezoelectric load cells so that all of the vibrational forces emitted by the reaction wheel assembly are transmitted through the sensing axes of the load cells.